Touch sensitive screens ("touch screens") are devices that typically mount over a display such as a CRT. With a touch screen, a user can select from options displayed on the display's viewing surface by touching the surface adjacent to the desired option, or, in some designs, touching the option directly. Common techniques employed in these devices for detecting the location of a touch include mechanical buttons, crossed beams of infrared light, acoustic surface waves, capacitance sensing, and resistive membranes.
For example, U.S. Pat. No. 4,484,179 discloses an optically-based touch screen comprising a flexible clear membrane supported above a glass screen whose edges are fitted with photodiodes. When the membrane is flexed into contact with the screen by a touch, light which previously would have passed through the membrane and glass screen is trapped between the screen surfaces by total internal reflection. This trapped light travels to the edge of the glass screen where it is detected by the photodiodes which produce a corresponding output signal. The touch position is determined by coordinating the position of the CRT raster beam with the timing of the output signals from the several photodiodes.
U.S. Pat. No. 4,782,328 on the other hand, relies on reflection of ambient light from the actual touch source, such as a finger or pointer, into a pair of photosensors mounted at corners of the touch screen. By measuring the intensity of the reflected light received by each photosensor, a computer can calculate the location of the touch source with reference to the screen.
Touch screens that have a transparent surface which mounts between the user and the display's viewing surface have several drawbacks. The transparent surface may cause multiple reflections on the viewing surface, produce glare and reduce the contrast ratio between displayed segments and the display background. These problems are greatest with LCDs that rely solely on absorption of ambient light to differentiate displayed dots or segments from a reflective background. Without additional lighting, the quality of an LCD image viewed through such an intervening surface is considerably reduced.